Creep of Prestressing Steels in Fire

Measured creep strain at high temperature for three different prestressing steels.

Our new paper, Creep of Prestressing Steels in fire, is now in press in the journal Fire and Materials (you can access it here; an open access version is to come). It is a great piece about prestressing steels used in Post-tensioned structures. We show how high- temperature creep is effected by the chemical composition of the steel. While it may seem obvious to some, the effect this can have for prestressing steel in high temperatures seen in real fires is astounding.

Consider several different prestressing steels, all behaving the same in ambient service temperatures, but when stressed and heated (700 MPa and 427C) as if they were in a fire, one fails after 5 hours, were the others fail below 1.5 hours. All likely due to the chemical composition from the production process of the steel (see the above figure). These tests are described in the paper (that and about 80 others). The same trends shows the same effect in simple strength tests at high temperature.

The paper also describes the use of digital image correlation to measure the strain at high high temperature. An innovated technique, but leading to important insights about true areas and strains as the steel fails in high temperature. A compiled video of a creep test is provided below which illustrates how quickly tertiary (third stage) creep takes effect before failure.

The above video shows the prestressing steel coated in a black and white speckle texture pattern. Image correlation is a powerful technique for measuring high temperature strains. Its a very simple process described below

Digital image correlation is an excellent strain measurement technology. This figure shows two images taken using image correlation techniques on a piece of prestressing steel. The top image illustrates time zero with an applied stress of zero. The second photo below shows the specimen at 100 seconds, with an applied stress state of 1950 MPa. The speckle pattern seen allows a software to track movement of the bar as it deforms (strains) between different pictures. The software tracks tiny patches. Think of them loke a box. The software can then be used to tell the user how much these patches move between the images.